1. Field of the Invention
The present invention relates to a semiconductor light-emitting device represented by light-emitting diode (LED) and the like, and a method for forming an electrode provided in the semiconductor light-emitting device.
2. Description of the Related Art
The semiconductor light-emitting device represented by LED and the like has advantages such as low power consumption, compact in size, high brightness, and long life, and thus is recently used for various applications. For example, the semiconductor light-emitting device has been used for a lighting system, as a substitute for incandescent lamp that consumes a large amount of electric power.
In addition, a semiconductor light-emitting device having a reflection layer capable of reflecting light exiting from a light-emitting layer in an electrode, in order to further increasing light extraction efficiency, is suggested. For example, in Japanese Unexamined Patent Publication No. 2002-26392, Japanese Unexamined Patent Publication No. 2008-41866, Japanese Unexamined Patent Publication No. 2011-66461, Japanese Unexamined Patent Publication No. 2006-80469, Japanese Unexamined Patent Publication No. 2006-93358, and Japanese Unexamined Patent Publication No. 2011-204804 (hereinafter, referred to as “publicly known documents 1 to 6”), a semiconductor light-emitting device using Al that has high reflectance and is inexpensive as a reflection layer is suggested.
However, Al has a low melting point and is a chemically active material, therefore, there are problems that the surface is susceptible to corrosion, and void and hillock, and migration are likely to occur in the interior. Also, Al is a material that is likely to interdiffuse when contacting with a dissimilar metal, and for example, when contacting with Au, an intermetallic compound of AuAl that is highly resistive and fragile is formed by interdiffusion, and it becomes a factor to damage the reliability of the semiconductor light-emitting device, such that the forward voltage of the semiconductor light-emitting device increases, the adhesion strength of the electrode deteriorates, or the like.
Au is often used as a wire connecting an external power supply and a semiconductor light-emitting device, and in order to favorably connect to this wire, Au is often used as a pad layer of the electrode. However, when both Al layer and Au layer are contained in the electrode, the problem of contact described above arises.
Therefore, for example, publicly known documents 1 to 6 suggest a semiconductor light-emitting device having an electrode in which direct contact between an Al layer and an Au layer is avoided by inserting between the Al layer and the Au layer a layer made of a material other than Al and Au.
Publicly known documents 1 to 5 suggest a semiconductor light-emitting device having an electrode with a structure in which a layer made of a material such as Ti, Pt, W, Ta, and Pd is inserted between the Al layer and the Au layer. However, in the electrodes of the semiconductor light-emitting devices described above, the side surface of the Al layer is exposed, thus the side surface of the Al layer may corrode. In addition, an Au layer may be unintentionally formed on the side surface of the Al layer, and an AuAl intermetallic that is highly resistive and fragile may be formed by an interdiffusion of Al and Au.
On the other hand, publicly known document 6 suggests a semiconductor light-emitting device having an electrode with a structure in which the top surface and side surface of an Al layer are covered with a W layer and the top surface and side surface of the W layer are further covered with an Au. However, in the electrodes of the semiconductor light-emitting devices, an Au layer is present not only in the upper side of the Al layer but also in the side of the Al layer, therefore, the portion where the Al layer and the Au layer are adjacent to each other is increased, and the possibility to generate interdiffusion of Al and Au is increased. In addition, in order to prepare an electrode of this semiconductor light-emitting device, multiple times of photolithography step and multiple times of film formation step are required, and the cost is increased as the manufacturing process is more complicated, thus it is not practical.